1. Field of the Invention
The present invention relates to an ink composition for an ink-jet printer, and more particularly, to a water soluble ink composition adopting a pigment as a coloring agent used in an ink jet printer, which is improved in properties such as water fastness, light fastness and storage stability, and which can be easily manufactured.
2. Description of the Related Art
Ink jet printing is most commonly performed by a thermal bubble ink jet printing or a piezoelectric bubble ink jet printing method.
According to the thermal bubble ink jet method, when ink is heated to above its boiling point by a heating unit of the print head, bubbles are generated. The bubbles are then grown to form larger bubbles. When such larger bubbles are formed, pressure is applied to the surrounding ink, and printing is achieved while the ink is discharged through an ink discharge hole due to the pressure.
In the piezoelectric bubble ink jet printing method, when a voltage is applied to a piezoelectric unit, a change in ink volume is effected. Pressure is generated by the change in ink volume, and this pressure as applied to the ink. Printing is achieved while the ink is discharged through an ink discharging hole due to the pressure.
An ink composition for an ink jet printer is composed of a coloring agent such as a dye or a pigment, water, and additives such as an absorbent. When a dye is used as a coloring agent, it is difficult to select a dye to be printed in a desired color, and the resultant ink has a limitation in use due to its poor water fastness and light fastness. When a pigment is used as a coloring agent, water fastness and the light fastness are improved as compared to when a dye is used.
When an ink composition is made using a pigment as the coloring agent, pigment particles tend to aggregate hydrophilically due to the van der Waals' force during a drying process. Such aggregation of pigment particles may block the nozzle or discharge holes of a printer head. Thus, it is very important to pre-grind the pigment in order to avoid such a phenomenon. Generally, as the particle size of the pigment particles is decreased, color strength, glossiness, brightness and transparency improve. That is, print resolution is enhanced.
However, even when the pigment agglomerate is ground, the ground pigment particles tend to aggregate again. Thus, it is necessary to perform a dispersion process for preventing reaggregation of the ground particles by minimizing the total surface energy. The pigment after the dispersion process can be stabilized by electrical stabilization, steric stabilization using a stabilizing agent, or electro-steric stabilization. Also, in the case of dispersing the pigment, the steric stabilization method has been commonly adopted since steric stabilization is affected less by electrolytes than the electric stabilization which is affected by whether or not the electrolytes are contained in a dispersion medium.
Conventionally, a polymeric dispersing agent has been used as a dispersing agent. The polymeric dispersing agent is a copolymer having both hydrophilic and hydrophobic groups, wherein the hydrophobic group stabilizes the dispersion of the pigment by an interaction with the pigment, and the hydrophilic group interacts with a water soluble solvent to provide steric stability.
However, due to its high large molecular weight, physical properties (e.g., viscosity) of the ink composition changes considerably even if the change in the amount of polymeric dispersing agent added is small. Thus, it is very difficult to accurately control the amount of dispersing agent during the manufacture of the ink composition. Also, even if the polymeric dispersing agent contains hydrophilic groups, mole fraction of the hydrophilic group to the whole compound is low, so that its solubility in water is poor. Thus, it takes a long time to dissolve the polymeric dispersing agent in water. Also, when water of the ink composition near a nozzle evaporates, the polymer is precipitated due to its lower solubility in water, thereby blocking the nozzle. In order to prevent such an undesirable phenomenon, a wetting agent is necessary. However, using a wetting agent extends the amount of time required to dry the ink on paper.